A conventional charge pump generally includes two current sources for charging and discharging an output node to generate a suitable control voltage to a following voltage-controlled oscillator (VCO) to generate an output clock signal. The two current sources are generally implemented by a P-type transistor and an N-type transistor, respectively, and drain-to-source voltages of the P-type transistor or the N-type transistor are generally designed larger to have lower noise. However, the larger drain-to-source voltages mean that the control voltage has a narrow range. If the charge pump is applied in a low-voltage application (i.e. low supply voltage), this narrow headroom issue may become worse, and the functions and noise of the charge pump may be worsened.